Polymerization activated monomer mechanism

Anionic polymerization of lactams was shown to proceed according to what is called the activated monomer mechanism. With bischloroformates of hydroxy-terminated poly(tetramethyleneglycol) and poly(styrene glycol) as precursors for a polymeric initiator containing N-acyl lactam ends, block copolymers with n-pyrrol-idone and e-caprolactam were obtained by bulk polymerizations in vacuum at 30 and 80 °C, respectively361. ... 

Activated magnesium, 72 835 Activated monomer mechanism, for lactide polymerization, 20 300 Activated sludge, 72 24... 

A variety of protonic and Lewis acids initiate the cationic polymerization of lactams [Bertalan et al., 1988a,b Kubisa, 1996 Kubisa and Penczek, 1999 Puffr and Sebenda, 1986 Sebenda, 1988]. The reaction follows the mechanism of acid-catalyzed nucleophilic substitution reactions of amides. More specibcally, polymerization follows an activated monomer mechanism. Initiation occurs by nucleophilic attack of monomer on protonated (activated) monomer (XXIV) to form an ammonium salt (XXV) that subsequently undergoes proton exchange with monomer to yield XXVI and protonated monomer. The conversion of XXIV to XXV involves several steps—attachment of nitrogen to C+, proton transfer from... 

The anionic polymerization of lactams proceeds by a mechanism analogous to the activated monomer mechanism for anionic polymerization of acrylamide (Sec. 5-7b) and some cationic polymerizations of epoxides (Sec. 7-2b-3-b). The propagating center is the cyclic amide linkage of the IV-acyllactam. Monomer does not add to the propagating chain it is the monomer anion (lactam anion), often referred to as activated monomer, which adds to the propagating chain [Szwarc, 1965, 1966]. The propagation rate depends on the concentrations of lactam anion and W-acy I lactam, both of which are determined by the concentrations of lactam and base. 

Polymerizations initiated by strong bases (R-, IIO, RO-) and tertiary amines (which are poor nucleophiles) proceed at much faster rates than do polymerizations initiated hy primary amines. Also, unlike the latter, where each polymer chain contains one initiator fragment (i.e., RNH—), these polymerizations do not result in incorporation of the initiator into the polymer chain. Polymerization proceeds by an activated monomer mechanism similar to that in the anionic polymerization of lactams. The reacting monomer is the NCA anion XLIV... 

NCA polymerization by secondary amines may involve the amine or activated monomer mechanisms or both mechanisms simultaneously. Unhindered secondary amines such as dimethylamine and piperidine react like primary amines, and polymerization occurs by the amine mechanism. Polymerization by slightly hindered amines such as diethylamine, N-methylbenzylamine, and di-n-propylamine involves a combination of the amine and activated monomer mechanisms. More hindered secondary amines, such as di-n-isopropylamine and dicyclohexylamine, react almost exclusively via the activated monomer mechanism. 

A second route (route B in Fig. 1) relies on an initiation process with an (meth)acryl hydroxyl compound and is adopted from the chemical ROP of lactones. The controlled character of these polymerizations ensures a virtually quantitative initiation and thus incorporation of hydroxy-functional initiator (e.g., acrylate) into the polymer chain. However, this is not automatically the case for lipase-catalyzed ROP due to the different mechanism. The latter follows an activated monomer mechanism in which the lipase activates any carbonyl group of a carboxylic acid derivative present in the system. It has recently been shown that acrylation using hydroxy-functional acrylate initiators like hydroxy ethyl(meth)acrylate (HEMA or... 

Blout observed that the rate of polymerization of y-benzyl glutamic acid NCA was approximately 100-times faster when initiated with sodium methoxide rather than a primary or secondary amine.19,101 He proposed the active monomer mechanism of NCA polymerization previously formulated by Ballard and Bamford,1111 which involves the extraction of the proton from the NCA nitrogen to give anion 3, followed by nucleophilic attack of this anion on the amino acid carbonyl of a second NCA (Scheme 2). The active monomer mechanism was further studied and substantiated by Goodman.112-141... 

Additionally, an NCA may simultaneously polymerize by more than one mechanism. The active monomer mechanism provides polymers whose molecular weight is not dependent on initiator concentration, and high molecular weight polymers are readily obtained. The normal or carbamate mechanism should provide one polymer molecule per initiator molecule and the initiator is incorporated into the polymer. 

The polymerization of an NCA may be initiated by any moderately strong base or by nucleophiles. Weak nucleophiles, such as water, alcohols, or primary amines, generally initiate polymerization by the normal (nucleophilic) mechanism or by the carbamate mechanism. Tertiary amines and strong bases, such as methoxide, initiate polymerization by the active monomer mechanism. Secondary or primary amines may initiate polymerization by any one or all of these mechanisms. More than one mechanism may be active at any one time and frequently a polymerization may begin by the active monomer mechanism and then, at a later stage, propagate by the normal mechanism. 16 ... 

We recently investigated a different route for the synthesis of poly(IB-h-f-CL) diblock and poly( -CL-fo-IB-fo- -CL) triblock copolymers by site-transformation of living cationic polymerization of IB to cationic ring-opening polymerization of -CL via the activated monomer mechanism [95]. 

Anionic polymerization of 67 by the activated monomer mechanism should occur with the selective cleavage of the CO—NH bond of the monomer to give a polyamide composed of kinetically controlled cis units (68c). However, the cis units isomerize to the thermodynamically more stable trans units (68t) through the proton abstraction from the methine group adjacent to the carbonyl group. This was ascertained by the isomerization experiment in which a polymer consisting of 92% cis unit and 8% trans unit was converted to one containing 40% cis unit and 60% trans unit when heated in dimethyl sulfoxide at 80 °C for 6 hours in the presence of 15 mol% potassium pyrrolidonate. 

The lack of anionic polymerizability of 69 can be ascribed to the instability of 71 due to a stereoelectronic effect. [9] One of the lone pair orbitals on the nitrogen atom of 71 is perfectly antiperiplanar to the C(l) —0(2) bond, which causes this bond to cleave fadly. Thus, as soon as 71 is formed, it is transformed to 72, so that anionic polymerization by the activated monomer mechanism does not take place. 

Polymerization of oxirane (and of its derivatives) by the mechanism of activated monomer is so far exclusively cationic and can be represented by schemes (27) and (28) of Chap. 4. In contrast to the ring-opening polymerization of lactams, both the classical and the activated monomer mechanisms are operating in this case. Conditions can be found where one or the other mechanism predominates [339]. 

As discussed already for cationic polymerization of oxiranes, cycliza-tion can be eliminated if polymerization is performed under the conditions at which the activated monomer mechanism operates. This approach was used for cationic polymerization of e-caprolactone and other higher lactones [191]. Thus, in the polymerization of e-caprolactone in the presence of ethylene glycol (EG) and (C2Hs)30 +, PF6- catalyst, linear increase of molecular weight with conversion was observed up to M 3000 and polymers with DP = [M]o/[EG]0 and relatively narrow molecular weight distribution (MJM 1.3) were obtained. No cyclic oligomers were detected in reaction products. Similar results were obtained for polymerization of 5-valerolactone and j8-butyrolactone. Kinetic studies of the AM polymerization of lactones have been reported [192]. 

In the polymerization of unsubstituted lactams, propagation proceeds mainly by the activated monomer mechanism ... 

The polymerization occurs by the activated monomer mechanism, which supposes a two-step mechanism involving the acylation of the lactam anion (NaL) by the AT-acyllactam end-group followed by a fast proton-exchange with the monomer. In bulk polymerization, the preformed AT-acyllactams or their precursors (so-called CIs) are introduced into the system in order to avoid the slow initiation step due to the absence of AT-acyllactam groups at the beginning of polymerization ... 

The mechanism of NCA polymerization with amine as initiator has also been discussed earlier by many researchers (1 ). "The activated monomer mechanism" proposed by Bamford and Szwarc is one of the most widely accepted mechanisms. Proton... 

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